Conventional projection televisions employ various types of lamps (white lamps such as a discharge xenon lamp, a metal halide lamp, or a halogen lamp) as their light sources, and spatial modulation devices such as a liquid crystal and a DMD (Digital Micromirror Device®) as their light bulbs. In recent years, a projection television employing a light emitting diode or a semiconductor laser is being put into production, aiming to obtain a long service life of the light source and an expansion of color reproduction range of an image.
In a projection television employing a light emitting diode or a semiconductor laser as the light source, a color image is obtained by driving light emitting elements each emitting a monochromatic light of red, green, or blue in such a manner that each of the light emitting elements emits a time-division pulsed light at such a high speed (a few hundred Hz to a few kHz) that the flickering is hardly perceived by human eyes. In addition, a projection television employing a DMD as the light bulb often uses a pulse-width modulation method as a gradation representing method, which represents the brightness with a length of a display time of each pixel (Patent Literature 1).
However, when representing the gradation by the pulse-width modulation method, an emission waveform from a light emitting element used as the light source needs to be a stable waveform without having a fluctuation between individual waveforms and a fluctuation due to a temperature change and a temporal change. Generally, it is possible to obtain a stable emission waveform with a constant emission intensity from a light emitting element such as a laser diode or a semiconductor laser by driving it with the rated current.
Furthermore, in a green region or a blue region, it is often the case that a sufficient light intensity can hardly be obtained only with a laser oscillation. Therefore, a technique of oscillating an infrared laser light, for example, and then converting the light into a visible light having a half wavelength is used, where the wavelength conversion is achieved by using a wavelength converting element such as an SHG (Second Harmonic Generation) element.
As a technique of controlling a light emitting element for a wavelength conversion using a wavelength converting element, there has been proposed a technique of measuring an emission intensity of a light from a semiconductor laser with a photodetecting element and controlling the light emitting element based on the measurement result (Patent Literature 2).